Discharge lamp auxiliary circuit with dI/dt switching control

Abstract

A variable frequency AC source drives a gas discharge lamp which is connected in parallel with the capacitance of a series resonant circuit. The polarity of the voltage applied to the resonant circuit is commutated at such times as the rate-of-change of current flow in the resonant circuit is at or near zero. The source frequency is thus maintained at or near the circuit resonant frequency to maintain high output voltage when the lamp is starting or reigniting. The polarity is also commutated at such times as the current flow from the source reaches a predetermined level. Lamp current is thus controlled in the running mode.

The invention claimed is: 
     
       1. A circuit for starting and operating a gas discharge lamp of the type including a capacitance operatively connected in parallel with a discharge lamp, an inductance operatively connected in series with said capacitance to form a series resonant matching network, a variable frequency alternating current source connected to couple power into said matching network, and means for automatically adjusting the frequency of said source to excite a resonance of said network; wherein, as an improvement, said means for automatically adjusting the frequency comprises: first circuit means connected to commutate the polarity of the voltage across said matching network at such times as the time derivative of current flow in said inductance is substantially equal to zero, whereby the frequency of said source is automatically maintained at or near a resonance of said matching network.   
     
     
       2. The circuit of claim 1 wherein said source is a solid state inverter circuit. 
     
     
       3. The circuit of claim 1 wherein said means for automatically adjusting the frequency further comprise second circuit means connected to commutate the polarity of the voltage on said matching network at such times as the current flow from said source equals a predetermined level. 
     
     
       4. The circuit of claim 3 wherein said source is a solid state inverter circuit. 
     
     
       5. The circuit of claim 4 wherein said inverter circuit comprises push-pull transistor elements. 
     
     
       6. The circuit of claim 5 wherein said inverter further comprises means for causing commutation of the polarity of an output voltage, said means including a switching element connected to interrupt emitter current flow in said push-pull transistor elements. 
     
     
       7. The circuit of claim 6 wherein said switching element comprises at least one transistor. 
     
     
       8. The circuit of claim 1 wherein said first circuit means includes means for determining that the voltage across said inductance is substantially zero. 
     
     
       9. The circuit of claim 1 wherein said first circuit means includes means for determining that the voltage across said inductance is near zero. 
     
     
       10. The circuit of claim 1 wherein said first circuit means is connected to commutate the polarity of the voltage across said matching network at such times as the time derivative of current flow in said inductance is near zero. 
     
     
       11. The circuit of claim 1 wherein the output power of said AC source periodically decreases to a level sufficient to extinguish said lamp. 
     
     
       12. A method of operating a gas discharge lamp comprising the steps of connecting a capacitance in parallel with said lamp;   connecting an inductance in series with said capacitance and said lamp to form a series resonant circuit;   applying a voltage to said series resonant circuit; and   commutating the polarity of said voltage at such times as the time derivative of current flow in said series resonant circuit is approximately equal to zero.   
     
     
       13. The method of claim 12 further comprising the step of commutating the polarity of said voltage at such times as the current flow into said series resonant circuit equals a predetermined level. 
     
     
       14. The method of claim 12 further comprising the step of sensing the voltage drop on said inductance to determine the value of the time derivative of current flow in said series resonant circuit.